I have a client running FM 13, with SMTPit Pro. They use Comcast Biz, and suddenly had a problem like this about 4 days ago. The fix for them was to change the port. This said, is it possible the OP's ISP has changed ports?

Messages (with the same SMTP settings in the Send Message script step) are only blocked when that script is included in a FMS15 scheduled script.The same Filemaker Server 15 environment can send Error & Warning messages without problems.